This invention relates to a process for manufacture of trimellitic acid anhydride from 1,2,4-trimethyl benzene, commonly known as pseudocumene, and more particularly relates to a method of recovering pure trimellitic anhydride from the reaction mass obtained by the liquidphase oxidation of pseudocumene by air or oxygen.
The process of this invention provides a commercial process for the manufacture of 4-carboxyphthalic anhydride through the catalytic liquid phase oxidation of commercially available 1,2,4-trimethylbenzene (pseudocumene) with air in the presence of acetic acid as reaction solvent, separation and recovery of crystalline trimellitic acid from the oxidation reaction effluent, thermal dehydration of trimellitic acid to its anhydride and separation and recovery of that anhydride from intermediate oxidation by-product and other oxidation by-product impurities by distillation and/or vaporization process steps.
Pseudocumene is oxidized with air mainly to a mixture of dimethylbenzoic acids in the presence of catalysis provided only by cobalt and/or manganese oxidation catalysts under liquid phase conditions using acetic acid as the reaction solvent. By the use of oxygen as oxidant and a combination of cobalt as metal oxidation catalyst and alpha-methylenic ketones as side chain oxidation initiator or promoter, pseudocumene is oxidized mainly to a mixture of 2-methylterphthalic acid and 4-methyl isophthalic acid in the presence of acetic acid solvent and under liquid phase conditions at atmospheric pressure. Catalytic liquid phase oxidation of pseudocumene with air can be accomplished in the presence of acetic acid solvent and the catalysis provided by the combination of heavy metal oxidation catalyst and a source of bromine as disclosed and claimed in U.S. Pat. No. 2,833,816. This oxidation method using a combination of heavy metal oxidation catalyst and a source of bromine to provide catalysis describes the production of 92 weight percent trimellitic acid filter cake product in a two hour reaction at 198.degree. C. (about 390.degree. F.) The theoretical yield of trimellitic acid from pseudocumene is 175 weight percent. However, the oxidation method of U.S. Pat. No. 2,833,816 has been developed to produce total trimellitic acid yields in the range of 135 to 161 weight percent or about 77% to about 92% of theory based on the pseudocumene hydrocarbon feed. By total yield of trimellitic acid is meant all of the trimellitic acid in the oxidation reaction effluent.
The more highly developed catalytic liquid phase air oxidation of pseudocumene by the method of U.S. Pat. No. 2,833,816 using the catalysis provided by the combination of heavy metal oxidation catalysts therein defined with bromine or a source of bromine under liquid phase oxidation conditions produces total trimellitic acid yields of 135 to 161 weight percent based on commercially available pseudocumene. But, even then, there are also coproduced trimesic acid, iso- and terephthalic acids, 4-methylorthophthalic acid, 2-methylterephthalic acid, 4-methylisophthalic acid and formyl phthalic acids in amounts as to present substantial problems in the recovery of high quality trimellitic acid, dehydration of trimellitic acid to its intramolecular anhydride and recovery of that anhydride.
Another problem in the manufacture of trimellitic acid through the oxidation of pseudocumene to trimellitic acid in the presence of acetic acid comes from the relatively high solubility of trimellitic acid in acetic acid. This solubility goes from about 1.0 pound per 100 pounds glacial acetic acid at 80.degree. F. to 6.5 pounds per 100 pounds glacial acetic acid at 220.degree. F. The presence of water in the acetic acid increases the solubility of trimellitic acid so that in aqueous acetic acid solvent having 82 to 85% acetic acid and 18 to 15% water by weight there are dissolved at 80.degree. and 220.degree. F. about 3.2 pounds and 16.5 pounds trimellitic acid per 100 pounds solvent. Ordinarily aqueous acetic acid of 90 to 98% (10 to 2% water) by weight is used in the oxidation as solvent not only because acetic acid of higher strength is more expensive to recover but also because the presence of 2 to 10% water by weight substantially eliminates oxidation induction. During oxidation of the methyl groups to carboxylic acid groups water is produced as a by-product and is generally retained through the removal of heat of reaction by condensing the acetic acid and water boil up from the liquid phase in the oxidation zone and returning to condensate to the oxidation zone. The aqueous acetic acid solvent in the effluent removed from the oxidation zone can contain about 10 to 25% water (90 to 75% acetic acid) by weight when the 90 to 98% aqueous acetic acid solvent is used in the weight ratios of 5 to 2 parts per part of pseudocumene. Thus at usual crystallization temperatures of 60.degree. to 120.degree. F. a substantial amount of trimellitic acid remains in solution.
For example, in Example II of U.S. Pat. No. 3,161,658 there is described the cooling to 100.degree. F. of an oxidation reaction effluent containing for each 500 parts acetic acid solvent 200 parts trimellitic acid and 50 parts of pseudocumene oxidation intermediates. There was recovered 135 parts crystalline trimellitic acid per 500 parts of acetic acid solvent. Thus, of the originally produced 200 parts trimellitic acid there was left in solution 65 parts or 32.5%. This appears to have been an oxidation of pseudocumene conducted in the presence of acetic acid solvent in the ratio of about 3.5 parts solvent per part of pseudocumene. Higher ratios of solvent to pseudocumene would have caused a greater proportion of the total trimellitic acid to remain in solution at 100.degree. F. For example, at a 5 to 1 solvent ratio 45% of the trimellitic acid produced would have remained in solution at crystallization and filtration temperatures of 100.degree. F.
U.S. Pat. No. 3,161,658 provides one technique for recovering the trimellitic acid remaining dissolved in the aqueous acetic acid mother liquor. This is done by adding the mother liquor to a pool of molten trimellitic anhydride (370.degree.-375.degree. F.) and flashing off water and acetic acid vapors and drawing off from the molten pool liquid in an amount equivalent to the weight of solids charged with the mother liquor. This liquid draw off is solidified, ground and dissolved in a dialkyl ketone or aromatic hydrocarbon (the ketone solution must be filtered to remove insolubles) and the solution is combined with anhydride from dehydrated 100.degree. F. filter cake. The aromatic hydrocarbon solution is filtered to remove an insoluble oily residue and the filtrate cooled to 75.degree. F. to precipitate trimellitic anhydride. This anhydride can be added to the anhydride from dehydration of 100.degree. F. first filter cake. By simple flashing at 6 mm Hg absolute there is recovered a trimellitic anhydride product of 95% anhydride content, 95% pure in yields of 85 to 90% based on the trimellitic acid produced by the oxidation. However, the ketone and aromatic hydrocarbon solvents are flammable and their foregoing uses although advantageous do present fire hazards.
Other problems in the recovery of trimellitic anhydride from trimellitic acid produced by catalytic liquid phase air oxidation in acetic acid solvent arises in the distillative and/or evaporative separation of trimellitic anhydride from the melt produced by dehydrating trimellitic acid. In this melt there is a substantial amount of iso- and terephthalic acids produced mainly as co-products of oxidation and some by decarboxylation of trimellitic acid when the dehydration is carried out at temperatures of 410.degree. to 428.degree. F. or higher. The literature reports that trimellitic acid is dehydrated to its anhydride at 216.degree. C. (about 421.degree. F.). But at 410.degree. to 428.degree. F. some decarboxylation takes place not to produce phthalic anhydride only but rather to produce mainly iso- and terephthalic acids. However, this decarboxylation can be substantially eliminated during dehydration by operating at about 335.degree. to 400.degree. F. with an inert gas sweep. This is disclosed and claimed in U.S. Pat. No. 2,971,011. The gas sweep is conducted with gas inert to trimellitic anhydride at 335.degree. to 400.degree. F. Nitrogen, flue gas, CO.sub.2, hydrocarbon vapors and even steam can be used as inert gas.
Such gas sweep dehydration does not eliminate the problem caused by the presence of oxidation by-products iso- and terephthalic acids. When either or both of isophthalic acid and terephthalic acid are present in the molten trimellitic anhydride to be recovered by distillative and/or evaporative techniques they are carried over with the trimellitic anhydride vapors after the amounts thereof in the molten anhydride bottoms reaches their saturation concentrations. This, of course, adversely affects the clarity and purity of recovered molten trimellitic intramolecular anhydride and the reactivity of the anhydride.
The intramolecular anhydride of trimellitic acid has become a commercial starting material for surface coatings having the desired properties of high thermal decomposition, high temperature insulating properties and good resistance to chemical attack and are substantially insoluble. These surface coatings are obtained from prepolymers prepared, for example, from trimellitic intremolecular anhydrides and polyamines. Because of the trifunctionality of the intramolecular anhydride the final surface coating product is a polyimide-amide. The intramolecular anhydride of trimellitic acid also has become a starting material for solid foams obtained by reacting an isothiocyanate among other reactants with the intramolecular anhydride. Air and heat drying points and enamels with hydrocarbon or water solvent vehicles are also prepared from the intramolecular anhydride of trimellitic acid. For most of these uses, trimellitic acid intramolecular anhydride of an anhydride purity of 98 to 99% is required.
For many commercial applications mentioned above color of the trimellitic anhydride has become an important specification. Highly colored brown, tan, or even yellow products may no longer be acceptable. Triethylene Glycol (TEG) color is a typical standard measure of this performance quality of trimellitic anhydride. In this method a reaction of the trimellitic anhydride with a 300% molar excess of triethylene glycol is carried out at 500.degree. F. (about 260.degree. C.) to produce a solution whose color is matched instrumentally with APHA color standards. Reaction time is sixty minutes. A typical commercial product must have a TEG color of 170 or less.
The problems that require solving are the recovery of trimellitic acid anhydride in yields above 85 to 90% based on trimellitic acid produced by catalytic liquid phase oxidation of pseudocumene with air in the presence of acetic acid solvent, the increase of recovery of trimellitic acid from the oxidation reaction effluent, an improved distillative and/or evaporative process for separating the intramolecular anhydride from the crude anhydride melt obtained by the dehydration of impure trimellitic acid, elimination of the fire hazards accompanying the use of dialkyl ketones or aromatic hydrocarbon extract solvents previously disclosed for advantageous use in increasing the recovery of trimellitic acid anhydride and the other problems before mentioned.
U.S. Pat. No. 4,587,350, incorporated by reference herein, discloses a process for the oxidation of pseudocumene to trimellitic acid by a catalytic oxidation of pseudocumene with air in the presence of acetic acid in a oxidation zone in the liquid phase with catalysts comprising zirconium, cobalt, and manganese and a source of bromine.
The process of this invention provides an integrated system for the commercial production of trimellitic acid anhydride.